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[DPRG] Handy Board and Encoders
Subject: [DPRG] Handy Board and Encoders
From: Mike McCarty
jmccarty at ssd.usa.alcatel.com
Date: Mon Sep 10 10:48:51 CDT 2001
On Wed, 5 Sep 2001, R. Bickle wrote:
> Jitter is an age old problem with optical encoders. You are right about the
> schmitt trigger Dan. There are two solutions as far as I know. Either add
> hysteresis, either with schmitt triggers or manually. (Sometimes, depending
> on the characteristics of the encoders you may need more hysteresis than a
> Schmitt trigger has.) The other alternative is of course to use quadrature
> encoders. There are several techniques for quadrature encoders which will
> totally eliminate jitter. (I have even had trouble with flourescent lighting
> activating the optointerrupters when they are just at the threshold.)
> Anyway, you can take the two channels of the quadrature signal and feed a SR
> flip flop. This will reduce the frequency by 1/2, but will ignore multiple
> counts from one channel. Also, you can take the quadrature signals and feed
> a D flip flop which is a simple way of indicating direction. I can post some
> schematics for hysteresis circuits if anyone is interested.
Sorry to jump in late like this. I've been working until midnight to
meet delivery. Delivered on Friday, so I'm catching up :)
It seems to me that a quadrature sensor should be completely
insensitive to jitter altogether, with or without hysteresis, if it is
read properly. No reading should be generated based on a transition on
one of the sensors unless there has been a transition on the other.
This can be enforced in software if not in hardware. Having a
transition on one arms the circuit to generate an interrupt when there
is a transition on the other. This does not sound like a lot of
hardware. Two flipflops and one gate (74xx74 and 74xx00 would do, I
should think).
Mike
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